:-))) it works! I replaced it to "return eval(s)" and the results look extremely well guess I should get one of this crystal balls myself. one minor issue is still left: correct me if I am wrong: result of 2/-3 is 0 (at least this is how it is defined in the testcase) In Jython 2.1.3: >>> 2/-3 -1 Anyway many thanks for your help. Without this would have taken forever.